(CNN) -- When Jamil Macias looked down at his new sports uniform, he felt embarrassed by the size on his tag -- he knew it was the largest one on his middle-school baseball team.

Still, Macias' friends called him a giant "teddy bear," and he had gotten used to being one of the bigger kids.

But by seventh grade, his lifestyle was changing dramatically because of his weight. After years of team sports, he started looking for excuses to skip practice and games. As his interest dwindled, he chose to drop out altogether -- first from baseball, then, soccer and finally basketball. From then on, the gain accelerated. As a junior in high school, he weighed more than 300 pounds.

While his peers were trying out the latest Abercrombie & Fitch fads, he was trying to find pants large enough to fit his 44-inch waist.

"I felt a little bit isolated from society," he said. "After school, people would get ready for sports practices, and I would go home and do homework."

Yet it wasn't until Macias vacationed in Hawaii at age 17 that he really grasped the reality of his size.

While looking at the digital photos from the trip -- initially intending to show them to friends -- he was shocked by his appearance. "I never took very many pictures of myself," he said. "When I looked at them, I was too embarrassed to post them online. ... I (thought), 'Whoa, this is what I look like?' "

And soon after, during a routine physical, the doctor's scale showed 313 pounds.

That number, and the Hawaii photos, convinced him that he had to change.

But resolving to lose weight was nothing new -- on previous visits, his doctor had talked to him about going on a diet or getting a referral for a dietitian. Throughout high school, he'd sporadically headed to the gym, aware of his expanding size, except the habit never stuck.

But now he had motivation. Macias went home that night and began exercising in his garage to avoid being seen at a gym.

Unfortunately his third session on the elliptical ended badly; the strain of his weight broke the machine almost in half and landed him on the floor. His family wasn't there to witness it, but he felt humiliated when he thought about going inside to face them.

"I was really embarrassed because I had never thought in my mind that I would be big enough to break an exercise machine," he said.

So he began to work out by walking at night. After the disastrous session with the elliptical, he was careful to keep a cover between himself and any potential stares.

It was on one of his first evening jogs that Macias specifically remembers feeling hurt as teenagers in a truck drove by and called him, "Fatty."

"Stuff like that wasn't common, but it did happen," he said. "I used that as a motivator to keep going -- I wasn't going to let something like that stop me."

Macias pushed himself further each night, progressing from walking to running, in addition to eliminating fast food and replacing processed, sugary foods with fruits and vegetables. His family -- especially his mother -- was supportive, keeping track of the number of pounds lost right along with him.

"I did the research on my own," he said. "I never followed a specific diet or book. ... I explored other (food) options that I knew were better substitutes."

He starts out breakfast with protein-rich dairy, such as cottage cheese and yogurt. Lunch and dinner always include salad and vegetables in addition to an entrée that's high in protein.

"I didn't have a set amount of how much I wanted to lose -- I just wanted to keep going until I felt good about myself," he said.

After about three years, Macias had dropped nearly 130 pounds and weighed in at 185 in September 2010. "I felt really good," he said. "It took me awhile to really understand my body and know what I can do to it, change it, help it grow and become stronger."

Moving forward

Every day of the week at 5:30 a.m., Macias, now 22, wakes up to hit the gym, usually running for about half an hour on the treadmill and then lifting weights.

Since graduating from college in April, Macias has gained 10 pounds of weight from training to build up muscle. He ran a half marathon in February and is training to run another in August.

"We can all make time to exercise," he said. "You have to stay motivated and positive and pretty much eliminate the excuses."

Besides his physical appearance, Macias said, his lifestyle changes have affected how he interacts with people.

During high school, he would have described himself as reclusive -- shy and unengaged.

But thanks to a healthier lifestyle, Macias said he feels a higher self-esteem and -- if placed in a group -- would immediately identify himself as the leader, filled with confidence he once lacked.

"That was really the theme that kept me going -- I wanted to be more engaged in life, and I knew that (losing weight) would help me do it," he said.